1. Field of the Invention
The present invention relates to the formation of integrated circuit devices and, more particularly, to the formation of integrated circuit devices using ion implantation processes that exhibit reduced levels of cross-contamination.
2. Description of the Related Art
Doping of high density integrated circuit devices is accomplished by ion implantation for a variety of reasons. Among the more important characteristics of ion implantation are the ready availability of masking techniques for controlling the lateral extent of the doped region formed by the implantation, the ability to control implantation dosages and energies, and the relatively high speed of the implantation process. These desirable characteristics make ion implantation a more manufacturable process than other doping techniques in most applications. Ion implantation is commonly used for forming P-type or N-type isolation wells, FET source/drain regions, channel stop implantations, threshold adjust implantations, anti-punchthrough implantations, and other laterally defined doped regions. It is difficult to achieve the desired levels of lateral definition using processes other than ion implantation. Because of its comparatively low cost and flexibility, ion implantation is also utilized when other doping techniques could be used. For example, ion implantation is commonly used in creating doped surface layers on entire wafers that have either a conductivity type different than the rest of the wafer or that have a doping level that is higher than the concentration of the rest of the wafer. Although diffusion could readily be used for this application, ion implantation is generally preferred.
Ion implantation has certain drawbacks for the manufacture of integrated circuit devices, however. Of present concern is the fact that ion implantation can introduce a variety of contaminants into the region being implanted. Highly excited ions are typically present within the ion implanter and the highly excited ions are incident on surfaces within the implanter other than the surface of the workpiece being implanted. Affected surfaces of the implanter include, for example, the shutter that separates the ion beam line from the ion implantation chamber and the clips that hold the workpiece in place during the implantation process. In the past, both the shutter and the workpiece clips have had metal surfaces so that the incident beam of implantation ions would sputter metal atoms from their surfaces that could be deposited onto the surface of the workpiece or even be implanted into the workpiece. Such metallic contamination is undesirable and steps have been taken to eliminate this contamination mechanism, such as coating the shutters and the clips with an inert or non-metallic, non-sputterable material. These efforts have reduced the level of metallic contamination observed in modem ion implantation systems, although this contamination source continues to be of concern.
There are other contamination mechanisms that act in ion implantation processes. Studies have suggested that one of the most important contamination mechanisms in ion implantation is cross-contamination. Cross-contamination occurs in different ways, but a common mechanism is for a first type of dopant ion to be implanted into a first workpiece during a first implantation process. During this first implantation process, dopant ions of the first type are implanted into the ion beam line, into the shutters and other apertures, into the workpiece supports, and other portions of the implantation chamber. In a second, subsequent implantation process utilizing dopant ions of a second type, the second dopant ions are incident on all of the surfaces of the implanter into which the first dopant ions were implanted in the first implantation process. The first dopant ions can be sputtered from the surfaces of the implanter and may be accelerated by collisions with the second dopant ions to an implantation energy. Consequently, a fraction of first type dopant ions may be deposited on the surface of the second workpiece or may be implanted into the second workpiece during the implantation of the second type dopant ions. This contamination process can affect the uniformity of dopants over the surface of a workpiece to a significant extent. For example, the variations in dopant concentrations can be sufficiently large to produce measurable variations in the surface conductivity of the implanted wafer. The levels of reported variations in surface conductivity may be on the order of less than one percent up to as much as ten percent. Another effect of cross-contamination is variation in junction depth. Earlier implanted ions can be sputtered from the surfaces of the implanter and can be implanted into contact regions, isolation wells, or other junctions. If the earlier, contaminant dopant ion is a comparatively light, rapidly diffusing species and the second, intended dopant is a comparatively heavy, slowly diffusing species, an activation anneal could cause the earlier, contaminant dopant to diffuse rapidly through the junction region created by the second dopant ion. The activation anneal causes the more rapidly diffusing (earlier) dopant to extend deeper into the substrate than the junction created by the implantation of the desired second dopants in a manner that may alter the electrical characteristics of the junction and may create a more graded junction than is desired.
These and other effects associated with cross-contamination can significantly harm the performance of the integrated circuit devices manufactured using ion implantation processes. Moreover, the above-described cross-contamination processes are not limited to successive implantations within an implanter. Reports indicate that exposed surfaces within implanters become saturated with implanted ions after only a short period of use. There are accordingly always impurity ions within the exposed surfaces of the implanter which could be sputtered from the exposed surfaces and which could be implanted into a workpiece in a cross-contamination process. Some of the problems associated with cross-contamination might be reduced if only a single dopant impurity were used in a particular piece of implantation equipment. Even if such a solution were practical, however, it may not provide satisfactory results. It would be expected that a single species implanter would exhibit improved performance with respect to junction depth variations. No comparable improvement would be expected for the two-dimensional uniformity of dopant concentration.